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RS485 topology and termination
Twisted pair cable is used as a wire in the RS485 bus (A and B). The RS485 bus has a straight
line topology with a maximum length of 1200 m. Straight line topology is recommended, and when
we can't avoid a branch, let's try not to exceed 10 m. Star and ring topology should be avoided.
Terminating resistors (T) must be activated at the extreme ends of the bus (fig. 3).
fig. 3
Important!
Connections via terminals A and B are sufficient for RS485 communication. We
recommend potential equalization and connection of the common terminals of all units. This
avoids potential differences that can lead to communication errors or make communication
completely impossible.
Important!
Terminating resistors are used to avoid signal reflections. They are connected in
parallel between terminals A and B of the RS485 bus at its extreme points. To connect the
terminating resistor in the RS485+ IoT card, the jumper must be shorted on the TERM pins (see
figure 1, element 3).
Important!
The quality of the cables used and their diameter also affects the quality of data
transmission. It is recommended to increase the wire diameter as the bit rate increases,
because as the wire diameter increases, its resistance decreases.
Important!
Try to don't unwinding your twisted pair more than one or two inches. This may
be difficult with shielded cables, but it is worth taking care of the cabling quality.
